Touch inputs on a touch surface are widely used as an input methodology. Touch inputs may be best known in conjunction with appliance control panels, smart phones and other handheld devices. However, touch screens and touch inputs are gaining widespread acceptance as a user interface over a wide variety of applications. It is also important that a touch surface have an appropriate sensitivity to touch. If the surface is “too sensitive,” it may be susceptible to noise, or it may register false touches. If the surface is “too insensitive,” it may not accurately register desired touches, or fail to register a touch altogether.
Capacitance sensing has been well established as a method of detecting a variety of stimuli, including touch inputs, with an improved sensitivity to touch. Capacitive sensors normally require at least one electrical element typically referred to as an electrode, element, or plate. In some instances, there may be one, two or more networks of capacitive electrodes, elements or plates. These elements are geometrically designed to cause the formation of a net electric field in both a non-stimulus state as well as a stimulus state.
Many methods of generating an output based on the difference between a non-stimulus state and a stimulus state are known in the art. According to one known method, a capacitive sensor is provided which includes at least one electrode. The electrode can be geometrically designed to detect a change in capacitance due to a stimulus, e.g., the presence of a nearby object. A measuring circuit converts the output of the electrode into a voltage, current, frequency, period or pulse width that is linearly or non-linearly proportional to the change in capacitance of the electrode. The electronic measuring circuit then evaluates the change in capacitance against a predetermined reference value. A change in capacitance in excess of the predetermined reference value indicates the proximity of the object to the capacitive sensor.
Problems with the aforementioned method include: (1) inability to compensate for dielectric variations among different objects; (2) susceptibility to environmental conditions; (3) inability to compensate for manufacturing tolerances and variations in component materials; and (4) when configured as a touch sensor, inability to detect an input at the touch surface without respect to whether a finger is gloved or not. In an attempt to overcome the aforementioned problems, known measuring circuits typically average the capacitive output over a variety of non-stimulus conditions to achieve a desired reference value. The resulting reference value can then be used to determine if there is sufficient proportional change (as described above) to indicate the presence of a valid stimulus. However, the determination of an averaged or compensated reference value often requires processing in software or a devoted microcontroller, which in turn can add cost and unneeded complexity to the overall system. Even with averaging algorithms, some variation, such as increased touch substrate thickness, can not be adequately compensated for to eliminate sensitivity variations.
For example, consider a capacitive sensor 20 provided to sense a human finger 22 against a given substrate 24 as shown in FIG. 1. As the finger 22 is brought towards the substrate 24, it approaches an electrode 26 with varying degrees of proximity 28, 30, 32, 34, 36. Once the finger 22 has approached and moved to a first distance 28, the electrode 26 and the corresponding measurement circuit 38 will attempt to detect the stimulus as described above with an output that is proportional to the stimulus. At this first distance, the finger is effectively far enough from the electrode 26 that there is effectively no stimulus condition. As the finger 22 approaches the substrate, however, the measurement circuit 38 evaluates the degree of stimulus against the predetermined reference value to determine the presence or absence of a valid stimulus. To account for variations in the environment, several reference values can be set to best identify a valid stimulus event. However, such a system would have to account for a number of factors, including the composition and variation of the construction materials, the variances in thickness of the different materials, the manufacturing processes and variances of the bonding of these materials, the dimensional tolerances of the electrode(s) dimensions, and the sensitivity preferences of the user. Alternatively, prior art systems can average the capacitive output over a variety of non-stimulus conditions to achieve a single, compensated reference value. However, the determination of a compensated reference value can be costly and impractical, requiring processing in software or a devoted microcontroller, and potentially delaying the response time of the associated capacitive sensor, and ultimately may not be able overcome sensitivity variations or detect valid stimuli such as a gloved or ungloved finger when the capacitive sensor is configured as a touch sensor.